LEC
06.21
Determination of Faraday’s constant
Related concepts
Electrolysis coulometry, charge, Faraday’s laws, Avogadro’s
number, general equation of state for ideal gases.
Principle
Faraday’s laws of electrolysis describe the correlation between
the amounts of substances transformed in the reactions at the
electrodes and the charge applied (amount of electricity).
Faraday’s constant, which appears as a proportionality factor,
can be determined experimentally from this dependence.
Tasks
Determine Faraday’s constant from the dependence of the volumes of hydrogen and oxygen evolved on the charge applied in
the hydrolysis of dilute sulphuric acid.
Equipment
Power supply, universal
Digital multimeter
Electrolysis apparatus after Hofmann
Platinum electrode, protective sleeve, d = 8 mm
On/Off switch
Connecting cord, l = 250 mm, red
Connecting cord, l = 500 mm, red
Connecting cord, l = 750 mm, blue
Retort stand, h = 750 mm
Right angle clamp
Universal clamp
Stopwatch, digital, 1/100 s
Weather monitor, 6 lines, LCD
Precision balance, 620 g
13500.93
07134.00
44518.00
45206.00
06034.01
07360.01
07361.01
07362.04
37694.00
37697.00
37715.00
03071.01
87997.01
48852.93
1
1
1
2
1
2
1
1
1
4
3
1
1
1
Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3062101
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LEC
06.21
Determination of Faraday’s constant
Beaker, 600 ml, short
Funnel, glass, do = 80 mm
Pasteur pipettes
Rubber bulbs
Wash bottle, 500 ml
Sulphuric acid, 0.5 M, 1 l
Water, distilled, 5 l
36015.00
34459.00
36590.00
39275.03
33931.00
48462.70
31246.81
1
1
1
1
1
1
1
Set-up and procedure
Set up the experiment as shown in Fig. 1.
Prepare the 20 % sulphuric acid solution required for the experiment by carefully adding 62 g of concentrated sulphuric acid to
238 g of distilled water in a 500 ml beaker.
Open the cocks of the electrolysis apparatus and fill in 200 ml of
the sulphuric acid solution through the levelling vessel. Eliminate
air bubbles in the apparatus by gently knocking on the tubes. Fill
the measuring tubes exactly by raising the levelling vessel, then
close the stopcocks.
Apply about 400 mA for just a few minutes to carry out electrolysis and so saturate the solution in the apparatus with the
evolved gases. Turn off the power supply and open the stopcocks to again completely fill the tubes with solution, then close
the stopcocks.
In the first part of the experiment, carry out electrolysis at a constant amperage of between 200 and 300 mA for 10 minutes,
starting the stopwatch at the same time as current is applied.
Interrupt the supply of power each minute and read off the volumes of gases that have been evolved. To do this, adjust the
height of the levelling vessel so that the meniscus in the vessel
is level with the meniscus in the tube being measured. Record
the time and the gas volumes in a Table. When the tenth readings have been taken, select a suitable scale and plot a graph of
the gas volumes measured against time.
In the second part of the experiment, subject the solution to
electrolysis at three different amperages (70, 140, 210 mA), each
for the same length of time (between 5 and 10 minutes). Read off
the gas volumes when the chosen time has elapsed and plot
them in a graph. Re-fill the tubes with acid prior to each measurement.
Determine the atmospheric pressure and room temperature.
Theory and evaluation
If an electric current I is forced by a direct source over a time
period t, changes occur in the given electrolysis system due to
anodic oxidation and cathodic reduction.
Oxygen is formed at the anode and hydrogen at the cathode.
Oxidation (at the anode):
6 H2O
O2 + 4 H3O+ + 4 e-
S
where
n
q
zR
F
Quantity of substance
charge
Number of transferred electrons per formula conversion
Faraday’s constant; the product of Avogadro’s number
NA and the unit charge e
The Faraday constant represents the quantity of electricitity that
is required to liberate or deposit a gram-equivalent (= NA monovalent ions). The same quantity of electricity always liberates or
deposits equivalent quantities of substances from different electrolytes (Faraday’s second law).
Under the proper conditions (low pressure, sufficiently high temperature), the quantities of gas n evolved in the two electrode
reactions can be expressed as the corresponding gas volumes
V using the general equation of state for ideal gases:
n!
where
R
p
T
pV
RT
(2)
Universal gas constant; 8.31441 J · K-1 . mol-1
Pressure
Absolute temperature
With equation (1) we obtain
V!
!q!RT
zR p F
(3)
According to this relationship, linear correlations (Fig.2) result for
the function V = f(q) which has the following slope:
RT
¢V
!
¢! q !
zR p F
Fig. 2:
(4)
Correlation between the transferred charge and the
evolved volumes of hydrogen and oxygen in the electrolysis of diluted sulphuric acid (T = 296.05 K and p =
100.4 kPa)
Reduction (at the cathode):
4 H3O+ + 4 e-
S
2 H2 + 4 H2O
The sum of these is:
2 H2O
S
2 H2 + O2
This experiment shows that the gas volumes evolved are proportional to the amperage and to the time. Faraday’s first law
can be derived from this:
n!
2
P3062101
!q!
I·t
!
zR F
zR F
(1)
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEC
06.21
Determination of Faraday’s constant
From the slopes of the graphs obtained for hydrogen (zR = 2) and
oxygen (zR = 4) Faraday’s constant can be calculated, if p and T
have been determined.
The graphs of the measured values can be very simply evaluated using “Measure“ software. There is no license fee for this
software when it is used for evaluating and graphically representing measured values. It as available under the download-file
URL www.phywe.de.
Data and results
An electrolysis performed with I = 300 mA in steps of t = 120 s
at T = 296.05 K and p = 100.4 kPa provided the linear correlations between the applied charge q = I · t and the thus-evolved
volumes of hydrogen and oxygen presented in Fig.2.
Using the slopes obtained from compensation calculation
(∆V (H2) / ∆ ! q ! = 1.298 · 10-7 . m3 . As-1 and
∆V (O2) / ∆ ! q ! = 0.640 · 10-7 . m3 . As-1 ),
the Faraday constant is determined to be F = 94477 As · mol-1
resp. F = 95739 As . mol-1.
Literature value: F = 96485 As · mol-1.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3062101
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LEC
06.21
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P3062101
Determination of Faraday’s constant
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen